Before his heart transplant, Muñoz "had been cleared to return to work" with the use of an implanted medical device. He also said shortly after his heart transplant that he planned to "participate in key meetings" before returning full time.

Muñoz had been president of CSX(CSX) Corp, a railroad company. He took over at United(UNITED CON) from Jeff Smisek, who left amid a federal investigation into whether it created a flight from Newark to South Carolina solely to benefit the former chairman of the Port Authority of New York and New Jersey.

United had other problems too: It ranked low in customer satisfaction, employee morale was hurting, and United's stock was on a downward slide.

In a statement Sunday, Muñoz said the company is "performing well."

"[O]ur team has been focused on our employees, improving the operation and the customer experience, and the results are starting to show," he said. "United's 2015 earnings were one of the best in the Company's history."